1. Field of the Invention
The present invention relates to optical security devices, especially those designed for security printing and brand authentication, and more particularly to personalized or serialized optical security devices and the on-demand digital printing and application thereof.
2. Background Information
Optical security devices are useful authentication tools that help to protect against the fraudulent reproduction of the objects or documents to which they are applied. The normally high cost and difficulty in reproducing certain optical security devices makes them well suited for this purpose. For example, images may be designed such that they are nearly impossible to copy, even when using an advanced color photocopying techniques. Additionally, optical security devices can be incorporated in or affixed to a document or object such that their removal will sufficiently alter the image, rendering it unusable. Several types of optical security devices currently exist. Typically, these include holographic images, kinegrams, stereograms, optical variable devices (xe2x80x9cOVD""sxe2x80x9d), images produced from metallic or pigmented foils, and the like.
One type of optical security device used for authentication purposes is the transmission hologram. Typically, transmission holograms are transparent and allow light to pass through from behind to reconstruct the holographic image. Often, however, when a transmission hologram is mounted to the surface of an opaque object a specular metal layer is used as a backing material. Reflected light then bounces off the metal layer and effectively lights the hologram from behind. These embossed or xe2x80x9cshinyxe2x80x9d holograms are a specific type of transmission hologram, and are the type of hologram typically used for security printing and brand authentication on items such as credit cards and software packaging. An embossed hologram is popular because it may be reproduced relatively quickly and inexpensively in large quantities. This type of hologram is typically provided, in quantity, as part of holographic foil.
A holographic foil commonly carries a series of identical embossed holographic image. The process of creating a holographic foil is fairly standardized. First, a laser light source and optical components are used to create a diffraction light pattern. This pattern is used to expose a film, which creates a photosensitive master. Base shim stock comprised of zinc and or other metals, coated with a photosensitive acid resist like a photo-polymer emulsion, is exposed to a light source through the master. The unactivated resist is then removed through a wash step. Next, the prepared shim is placed in an electroplating bath to add the materials that create the holographic image. Alternatively, the shim may also be created with etching (by removing material) using a photonegative master. Finally, the shim is used to emboss a topcoat (lacquer layer) and/or an aluminum layer prior to applying a sizing coat. The resulting holographic foil is normally used as a hot-stamping foil for foil blocking, where a run of several million copies is not uncommon.
Hot-stamping is a well-established method of transferring premade embossed holographic images to a substrate. Hot-stamping involves using a heated, image-shaped die under pressure to thermally transfer an image from a foil web. The heat and pressure from the die cause a pigment or a metal film to be released from a carrier and the holographic image to be transferred to the substrate. To transfer an image, the die is first heated, typically to between 250xc2x0-275xc2x0 F., and the die is then pressed against the holographic foil, typically with several hundred pounds of pressure, for a dwell time of approximately xc2xd second. Hot-stamping is adequate for repeatedly transferring the same shaped image to a series of substrates. However, a significant drawback with hot-stamping is that the fabricating of the die is time consuming and relatively expensive. Therefore, creating large quantities of unique or personalized holograms using this method is impractical.
Other types of foils, such as pigmented foils, metallic foils and foils with a high reflective index (xe2x80x9cHRIxe2x80x9d) coating, produce images that are difficult or impossible to photocopy and are thus particularly suited for generating optical security devices. Like the above-described holographic foils, these foils are also traditionally applied using hot-stamping or other similar means, and are therefore not particularly suited for producing on-demand personalized or serialized optical security images.
It is therefore an object of the present invention to provide a method and system for producing and applying individual optical security devices for security printing, brand authentication and other purposes. It is a further object to provide a method and apparatus for producing optical security devices that bear indicia that are uniquely associated with the owner of the item upon which the hologram is placed.
To accomplish the foregoing and other objects, features and advantages of the present invention I have provided a digitally-controlled thermal printing system that uses a digital print engine to transfer an optical security image from a thermal foil onto a substrate. Digital technology allows each applied image to be personalized or serialized and printed on demand.
The invention makes use of unique thermal foils designed for application by a digital print engine. Particularly, the thermal foils include a film carrier that resists distortion when subjected to the relatively high temperatures and pressures associated with the digital thermal printing process. More specifically, the thermal foils include a back-coating that comes into contact with the print head. The backcoating includes a lubricant that reduces the drag of a thermal print head, thus preventing the thermal foils from sticking to the thermal print head during printing.
The thermal foils used by the inventive system further include a top coat that resists distortion when subjected to the elevated temperatures (approaching 400 degrees F.) associated with the digital transfer process. The thermal foil preferably also includes a fast-acting yet aggressive thermally activated adhesive (size coat) that facilitates image transfer from the foil to a substrate.
Another feature of the inventive system is that the gauge of the film carriers employed by the thermal foils may be significantly thinner than typical hot-stamping foils. Thinner carriers are available because the employed foils do not need the strength or sturdiness required of hot-stamping foils. Thinner carriers allow for better heat transfer and thus quicker dwell times. Thinner carriers also allow for cooler print head temperatures, which helps to protect the foils from crazing. Additionally, thinner carriers allow for increased print speed.